THEMA: Problems when using passthrough-field

I am having issues using passthrough url fields. We are using Version 1.91+ Build 11379 and cannot update to 2.x at that time. We are also using closed access so tokens are initialised.
Here is the scenario:

1) customer logs in on website. once logged in the user has a session-token on the site which is always seen in the URL (GET-parameter)

2) after successful login survey is opened in an overlay (iframe), the website is still visible in the background. to open the survey the above mentioned session token is appended to the url as passthrough-field. this is needed so the user can continue to interact with the website once the survey is completed. this is what the call of limesurvey looks like

I have also added another question, hid it and gave the qcode sessionToken - just like the PASSTHRULABEL. Calling the survey like mentioned above I was assuming that this would have copied the value of my session token into the answer but that also did not work.

OK, so parsing of PASSTHRULABEL and PASSTHRUVALUE occurs only inside the END-URL. Reading the documentation I would have assumed that those placeholders could be put anywhere in the template but this is not the case.

qcode obviously does not work here because we are running 1.91, qcode is working in 1.92+ only.